Pneumatic tire

ABSTRACT

A pneumatic tire includes a tread portion, sidewall portions, bead portions, and has at least one carcass layer disposed between the pair of bead portions. The ratio SW/OD between the total tire width SW and the tire external diameter OD satisfies the relationship SW/OD≤0.3. A first region A is defined between a pair of first boundary lines (L 1 , L 1 ), second regions B are defined between a first boundary line (L 1 ) and a second boundary line (L 2 ), and third regions C are defined on the bead toe side of the second boundary lines (L 2 ). Defining SA, SB, and SC as the cross-sectional area (mm 2 ) of the first region A to the third region C, and defining the peripheral lengths (mm) of the first region A to third region C along the tire inner surface as a, b, and c, respectively, the relationship 7.5≤SA/a≤11.5 is satisfied.

TECHNICAL FIELD

The present technology relates to a pneumatic tire that is ideal forfitting to a passenger vehicle as standard, and more particularlyrelates to a pneumatic tire that can achieve reduced tire weight, aswell as steering stability and fuel economy performance.

BACKGROUND ART

Conventionally, various methods have been proposed for reducing therolling resistance of a pneumatic tire, in order to contribute to thefuel economy performance of a vehicle. In recent years in particular,with concern for the environment increasing, there is a demand forpneumatic tires to make a greater contribution to the fuel economy ofvehicles.

Reducing the air resistance around the tire by decreasing the totalwidth (SW) and the forward projection area of a pneumatic tire is knownas means for reducing the rolling resistance of a pneumatic tire (seefor example International Patent Application Publication No.WO2011/135774).

However, application of the above-described means results in a pneumatictire with a narrow total width and thus a narrow ground contact width.To maintain a certain load capacity, the outer diameter (OD) must thenbe increased. As a result, the ground contact length of such a pneumatictire is made long, and the ground contact width is made narrow.

If the ground contact width of a pneumatic tire is made narrow in thisway, the cornering force is reduced, and therefore there is apossibility that the steering stability will be reduced. Reducing thegroove area ratio of the tread portion in order to ensure steeringstability can be considered to solve this, but in this case, the fueleconomy cannot be sufficiently realized due to the increase in the tireweight. Therefore, even though the air resistance can be reduced basedon a narrow width and large diameter pneumatic tire, it is difficult torealize both steering stability and fuel economy performance.

SUMMARY

The present technology provides a pneumatic tire that can achievereduced tire weight, as well as steering stability and fuel economyperformance.

The pneumatic tire according to the present technology includes: a treadportion extending in the tire circumferential direction in an annularshape; a pair of sidewall portions disposed on either side of the treadportion; a pair of bead portions disposed on the inside in the tireradial direction of the sidewall portions; and at least one layer of acarcass layer disposed between the pair of bead portions; wherein

-   -   the ratio SW/OD of a total tire width SW and a tire outer        diameter satisfies the relationship SW/OD≤0.3;

in a tire meridian cross-section, the contour of the tread portion thatforms a tread profile includes a side arc located on the outermost sidein the tire width direction of the tread portion, and a shoulder arclocated on the inner side in the tire width direction of the side arc; apair of first boundary lines is defined passing through the intersectionpoint of the extension line of the side arcs and the extension line ofthe shoulder arcs and perpendicular to the tire inner surface; eachsidewall portion has a rim check line extending in the tirecircumferential direction; in a tire meridian cross-section, a pair ofsecond boundary lines is defined passing through the rim check lines andperpendicular to the tire inner surface; a first region is partitionedbetween the pair of first boundary lines; a second region is partitionedbetween a first boundary line and a second boundary line; a third regionis partitioned on a bead toe side of a second boundary line; whencross-sectional areas (mm²) of the first region through third region areSA, SB, and SC respectively, and peripheral lengths (mm) of the firstregion through third region along the tire inner surface are a, b, and crespectively, the ratio SA/a satisfies the relationship 7.5≤SA/a≤11.5;and

the groove area ratio GR in the ground contact region of the treadportion is 25% or less.

In the present technology, by providing the pneumatic tire with a narrowwidth and large diameter satisfying the relationship SW/OD≤0.3 for theratio SW/OD between the total tire width SW and the tire externaldiameter OD, the forward projection area of the pneumatic tire isreduced, and the air resistance can be reduced. Also, by setting thegroove area ratio GR in the ground contact region of the tread portionto 25% or less, deterioration in the cornering force and steeringstability due to the reduction in ground contact width can be prevented.In addition, when the pneumatic tire is partitioned with the firstboundary lines and the second boundary lines into the first region tothe third region, and the values are obtained by dividing thecross-sectional areas SA, SB, and SC of the first region to third regionby the peripheral lengths a, b, and c along the tire inner surface ofthe first region to third region, by satisfying the relationship7.5≤SA/a≤11.5 for the ratio SA/a, the volume of the first region of thepneumatic tire is reduced to the minimum necessary, the increase in tireweight associated with the reduction in the groove area ratio isminimized, and the rolling resistance can be reduced. In this way, thetire weight can be reduced and steering stability and fuel economyperformance can be achieved. As a result, vehicle fuel consumption isimproved, greatly contributing to resource saving and energy saving, andthe emissions of carbon dioxide from the vehicle can be reduced.

Preferably, the ratio SB/b satisfies the relationship 2.0≤SB/b≤6.0. Inthis way, the volume of the second region of the pneumatic tire isreduced to the minimum necessary, so the tire weight and the rollingresistance can be further reduced.

Also, preferably, the ratio SC/c satisfies the relationship4.0≤SC/c≤8.0. In this way, the volume of the third region of thepneumatic tire is reduced to the minimum necessary, so the tire weightand the rolling resistance can be further reduced.

Preferably, the grooves formed in the tread portion include at least onemain groove, and when the maximum groove depth of the main groove isGDmax, 3.0 mm≤GDmax≤6.0 mm. By setting the maximum groove depth GDmaxcomparatively small in this way, the cornering force is increased, andthe steering stability can be improved.

Preferably, at least one inclined reinforcing layer including aplurality of reinforcing cords inclining with respect to the tirecircumferential direction is disposed on the outer circumferential sideof the carcass layer in the tread portion. By providing the inclinedreinforcing layer in this way, the cornering force is increased, and thesteering stability can be improved.

Preferably, at least one circumferential reinforcing layer extendingalong the tire circumferential direction is arranged on the outercircumferential side of the inclined reinforcing layer. By providing thecircumferential reinforcing layer in this way, the cornering power isincreased, and the steering stability can be improved.

Preferably, the circumferential reinforcing layer is configured from acomposite material with reinforcing cords oriented in the tirecircumferential direction embedded in rubber. By increasing the in-planebending stiffness of the inclined reinforcing layer by adding thecircumferential reinforcing layer that includes reinforcing cordsoriented in the tire circumferential direction, the cornering power isincreased, and the steering stability can be improved.

Preferably, the reinforcing cords of the circumferential reinforcinglayer are organic fiber cords. By using light organic fiber cords as thereinforcing cords of the circumferential reinforcing layer, the weightcan be reduced, contributing to reduction in rolling resistance.

Preferably, an air penetration preventing layer with an air penetrationcoefficient of 50×10⁻¹² cc·cm/cm²·sec·cmHg or less is provided in thetire interior and/or on the tire inner surface along the carcass layer.In particular, preferably, the air penetration preventing layer isconstituted from a thermoplastic resin or a thermoplastic elastomercomposition in which a thermoplastic resin and an elastomer are blended.By providing the air penetration preventing layer with a lower airpenetration coefficient compared with a conventional air penetrationpreventing layer made from butyl rubber, the air penetration preventinglayer can be made thinner, so the weight can be further reduced. Notethat the air penetration coefficient is the value measured in accordancewith JIS (Japanese Industrial Standard) K 7126 “Testing method for gastransmission rate through plastic film and sheeting” under 30° C.temperature conditions.

The pneumatic tire according to the present technology is particularlysuitable for passenger vehicles. Here, pneumatic tire for a passengervehicle means pneumatic tires fitted as standard to passenger vehiclesexcluding those for emergency use, and excludes tires for emergency useor for racing.

In the present technology, the total tire width SW is the total width ofthe pneumatic tire including any designs located on the sidewallportions, when the pneumatic tire is assembled on a rim, inflated to 230kPa (discretionarily set internal pressure) to specify the dimensions ofthe pneumatic tire, and is in an unloaded state. The tire externaldiameter OD is the external diameter of the pneumatic tire at this time.Note that the internal pressure of 230 kPa as described above isselected for specifying the dimensions of the pneumatic tire such as thetotal tire width SW and the tire external diameter OD. All theparameters of the tire dimensions stated in this Specification arespecified under an internal pressure of 230 kPa and in the unloadedstate. Thus, it should be understood that inflating to an internalpressure of 230 kPa is not necessary for the application of the presenttechnology, and the pneumatic tire according to the present technologyinflated to an internal pressure in the typically used range exhibitsthe effects of the present technology.

The rim used in the present technology has a rim diameter compatiblewith the inner diameter of the pneumatic tire, and has a nominal rimwidth corresponding to the specified rim width Rm (mm) shown in Table 2that is the value closest to the value (Rm=K1×Sn) obtained from theproduct of the nominal tire section width Sn and the coefficient K1determined from the correspondence table (Table 1) based on the aspectratio of the tire assembled on the rim, in accordance with ISO4000-1:2001.

TABLE 1 Aspect ratio K1 20-25 0.92 30-40 0.90 45 0.85 50-55 0.80 60-700.75 75-95 0.70

TABLE 2 Nominal rim width Rm(mm) 3 76.2 3.5 88.9 4 101.6 4.5 114.3 5 1275.5 139.7 6 152.4 6.5 165.1 7 177.8 7.5 190.5 8 203.2 8.5 215.9 9 228.69.5 241.3 10 254

Also, in the present technology, the groove area ratio GR is apercentage (%) of the groove area within the ground contact region tothe total of the land portion area and the groove area within the groundcontact region. Here, the ground contact region is the region of groundcontact surface when the pneumatic tire is fitted to the rim asdescribed above, inflated to an internal pressure of 230 kPa, and placedin contact with a flat surface with a load equivalent to 80% of the loadcapacity applied. The ground contact width is the maximum width in thetire width direction within the ground contact region. The groundcontact length is the maximum length in the tire circumferentialdirection within the ground contact region.

In the present technology, the load capacity is determined based on ISO4000-1: 1994. However, in the case of sizes for which the load capacityindex is not set in this ISO standard, it shall be determined by aseparate calculation taking into consideration the standards of variousoverseas countries, and in this case, the load capacity shall becalculated based on the standard in each country. Therefore, in thepresent technology, the load capacity of each tire size is calculatedfrom the following calculation equation from “Calculation of LoadCapacity” in the Commentary to JIS-D4202-1994, which is the actual loadcapacity equation adopted in the JIS standards.X=K×2.735×10−5×P^(0.585)×Sd^(1.39)×(D_(R)−12.7+Sd)

where, X=load capacity (kg)

K=1.36

P=230 (=air pressure (kPa))

Sd=0.93×S1−0.637d

S1=S×(180°−Sin⁻¹(Rm/S))/131.4°

S=design cross-sectional width (mm)

Rm=rim width corresponding to the design cross-sectional width (mm)

d=(0.9−aspect ratio)×S1−6.35

D_(R)=reference value of rim diameter (mm)

In addition, in the present technology, the cross-sectional areas of thefirst region to third region are the areas projected in the tirecircumferential direction in a tire meridian cross-section. Therefore,if there are circumferential grooves extending in the tirecircumferential direction or lug grooves extending in the tire widthdirection on the tread portion, the lug groove part is included in thecross-sectional area but the circumferential groove part is excludedfrom the cross-sectional area.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a meridian cross-sectional view illustrating a completepneumatic tire according to an embodiment of the present technology.

FIG. 2 is a meridian cross-sectional view illustrating the pneumatictire of FIG. 1.

FIG. 3 is a cross-sectional view illustrating a magnified view of thetread portion of the pneumatic tire of FIG. 1.

FIG. 4 is a developed view illustrating a tread pattern of the pneumatictire of FIG. 1.

FIG. 5 is a cross-sectional view illustrating a modified example of thetread portion of the pneumatic tire of the present technology.

FIG. 6 is a cross-sectional view illustrating a modified example of thetread portion of the pneumatic tire of the present technology.

DETAILED DESCRIPTION

A configuration of the present technology will be described below indetail with reference to the accompanying drawings. FIGS. 1 to 4illustrate a pneumatic tire according to an embodiment of the presenttechnology. In FIG. 1, CL is the tire equatorial plane, and AX is thetire central axis.

As illustrated in FIG. 1, the pneumatic tire of the present embodimentis provided with a tread portion 1 extending in the tire circumferentialdirection in an annular shape, a pair of side wall portions 2 disposedon both sides of the tread portion 1, and a pair of bead portions 3disposed on the inner side of the side wall portions 2 in the tireradial direction.

As illustrated in FIG. 2, at least one carcass layer 4 including aplurality of carcass cords that extend in the tire radial direction islaid between the pair of bead portions 3, 3. Organic fiber cords ofnylon, polyester, or the like are preferably used as the carcass cordsconstituting the carcass layer 4. Annular bead cores 5 are embeddedwithin the bead portions 3, and bead fillers 6 made of a rubbercomposition are disposed on the outer peripheries of the bead cores 5.If necessary, the bead filler 6 is disposed on the outer circumferentialside of the bead core 5 in order to fill the gaps between the bead core5 and the carcass layer 4. The bead filler 6 may or may not be providedin this way, but preferably is provided in order to prevent faultsduring manufacture. However, when providing the bead filler 6, forreduction in weight, preferably, the cross-sectional area thereof ismade as small as possible. Also, an air penetration preventing layer 7is provided on the tire inner surface along the carcass layer 4. The airpenetration preventing layer 7 may be embedded in the tire interioralong the carcass layer 4, or it may be provided on both the tire innersurface and the tire interior.

At least one inclined reinforcing layer 8 including a plurality ofreinforcing cords inclined with respect to the tire circumferentialdirection is disposed on the outer circumferential side of the carcasslayer 4 in the tread portion 1. Preferably, steel cords are used as thereinforcing cords of the inclined reinforcing layer 8 but organic fibercords such as aramid, polyolefin ketone (POK), polyethyleneterephthalate (PET), and the like can also be used.

As illustrated in FIG. 4, a plurality of main grooves 11 extending inthe tire circumferential direction, a plurality of lug grooves 12extending in the tire width direction, and a plurality of sipes 13extending in the tire width direction are formed in the tread portion 1.A predetermined tread pattern is formed by the main grooves 11, the luggrooves 12, and the sipes 13. The main grooves 11 are grooves having awidth of 3 mm or more at the opening to the ground contact surface andhave a slip sign, and their cross-sectional shape can be U-shaped orV-shaped, for example. Besides circumferential grooves extending in thetire circumferential direction, the main grooves 11 may be lateralgrooves extending in the tire width direction, or inclined groovesextending at an inclination to the tire circumferential direction.

In the pneumatic tire as described above, the ratio SW/OD of the totaltire width SW and the tire external diameter OD satisfies therelationship SW/OD≤0.3. By providing the pneumatic tire with a narrowwidth and large diameter in this way, the forward projection area of thepneumatic tire is reduced, and the air resistance can be reduced. Notethat for practical use, the ratio SW/OD may have a lower limit value of0.15. Also, in reducing the width and increasing the diameter of thepneumatic tire, preferably, the total tire width SW is set in the rangefrom 125 mm to 185 mm, and the tire external diameter OD is set in therange from 650 mm to 850 mm.

Also, in the pneumatic tire as described above, in a tire meridiancross-section, a tread profile 10 that forms the profile of the treadportion 1 is formed by connecting a center arc located in the centerportion in the tire width direction of the tread portion 1 and having aradius of curvature Rc, a side arc located in the outermost side in thetire width direction of the tread portion 1 and having a radius ofcurvature Rs, and a shoulder arc located on the inner side in the tirewidth direction of the side arc and having a radius of curvature Rsh.The shoulder arc is an arc that defines the profile of the road contactsurface of the land portion located on the outermost side in the tirewidth direction of the tread portion 1, and the side arc is an arc thatdefines the profile of the side wall surface of the land portion locatedon the outermost side in the tire width direction of the tread portion1. The center arc and the shoulder arc may be a common arc, or they maybe different arcs. Another arc may be interposed between the center arcand the shoulder arc. The shoulder arc and the side arc may be connectedso as to contact each other, or they may be connected so as to intersecteach other. Another arc may be interposed between the shoulder arc andthe side arc so that the two are smoothly connected.

Here, as illustrated in FIG. 3, on both sides in the tire widthdirection of the tread portion 1, when straight lines are drawn passingthrough the point of intersection P of the extension line Es of the sidearc and the extension line Esh of the shoulder arc and perpendicular tothe tire inner surface, a pair of first boundary lines L1 formed of thestraight lines is defined. Note that when the shoulder arc and the sidearc are directly connected, the point of intersection P is positioned onthe tread profile 10.

On the other hand, as illustrated in FIG. 2, each sidewall portion 2includes a rim check line 21 on the tire outer surface extending in thetire circumferential direction. This rim check line 21 is formed forconfirming the fit of the tire to the rim, and is normally a ridge thatprotrudes from the tire outer surface. On a tire meridian cross-section,when straight lines are drawn passing through the rim check lines 21 ofeach sidewall portion 2 and perpendicular to the tire inner surface, apair of second boundary lines L2 formed of the straight lines isdefined.

A first region A is partitioned between the pair of first boundary linesL1, L1 a second region B is partitioned between a first boundary line L1and a second boundary line L2, and a third region C is partitioned onthe bead toe 31 side of the second boundary line L2. If thecross-sectional area (mm²) of the first region A, the second region B,and the third region C are SA, SB, and SC respectively, and theperipheral lengths (mm) along the tire inner surface of the first regionA, the second region B, and the third region C are a, b, and crespectively, the pneumatic tire is configured so that the ratio SA/asatisfies the relationship 7.5≤SA/a≤11.5.

In the pneumatic tire as described above, by having the ratio SA/asatisfy the relationship 7.5≤SA/a≤11.5, the volume (the substantialaverage thickness) of the first region A of the pneumatic tire isreduced to the minimum necessary, so the tire weight can be greatlyreduced without loss of tire performance such as the wear resistance,and accordingly the rolling resistance can be greatly reduced. Here, ifthe ratio SA/a of the first region A corresponding to the tread portion1 is smaller than 7.5, the wear resistance will be reduced, andconversely if larger than 11.5, the weight reduction effect will beinsufficient.

In the pneumatic tire as described above, the ratio SB/b may satisfy therelationship 2.0≤SB/b≤6.0. In this way, the volume (the substantialaverage thickness) of the second region B of the pneumatic tire isreduced to the minimum necessary, so the tire weight can be greatlyreduced without loss of tire performance such as the cut resistance, andaccordingly the rolling resistance can be greatly reduced. Here, if theratio SB/b of the second region B corresponding to the sidewall portion2 is smaller than 2.0, the cut resistance will be reduced, andconversely if larger than 6.0, the weight reduction effect will beinsufficient.

In the pneumatic tire as described above, the ratio SC/c may satisfy therelationship 4.0≤SC/c≤8.0. In other words, by reducing the number ofwire windings of the bead core 5, reducing the cross-sectional area ofthe bead filler 6 or reducing the thickness of the rim cushion rubberlayer, the ratio SC/c may be reduced as much as possible. In this way,the volume of the third region C of the pneumatic tire is reduced to theminimum necessary, so the tire weight and the rolling resistance can begreatly reduced, without affecting the fitting characteristics, inparticular, the rim disengagement resistance. Here, if the ratio SC/c ofthe third region C corresponding to the bead portion 3 is smaller than4.0, the fitting characteristics will be poorer, and conversely iflarger than 8.0, the weight reduction effect will be insufficient.

Note that the appropriate ranges of the cross-sectional area SA of thefirst region A, the cross-sectional area SB of the second region B, andthe cross-sectional area SC of the third region C vary greatly dependingon the tire size. However, by defining the ratios SA/a, SB/b, and SC/cfrom the values obtained by dividing these cross-sectional areas SA, SB,and SC by the peripheral lengths a, b, and c of the respective regions,the above action and effect can be expected regardless of the tire size.

In the pneumatic tire as described above, the tread pattern formed byvarious grooves including the main grooves 11, the lug grooves 12, andthe sipes 13 is formed on the tread portion 1, but the groove area ratioGR in the ground contact region TCW of the tread portion 1 is set to 25%or less. By setting the groove area ratio GR in the ground contactregion TCW of the tread portion 1 in this way, sufficient land portionin actual contact with the ground can be ensured, the reduction incornering force and the degradation in steering stability can beprevented, even though the ground contact width has been reduced by thenarrow width and large diameter of the pneumatic tire. Also, even whenthe groove area ratio GR has been set wide, the volumes of the firstregion A through third region C of the pneumatic tire are reduced to theminimum necessary based on the ratios SA/a, SB/b, and SC/c as describedabove. Therefore, the increase in the tire weight associated with thereduction in the groove area ratio GR can be minimized and the rollingresistance can be reduced. In this way, the tire weight can be reducedand steering stability and fuel economy performance can be achieved.Here, if the groove area ratio GR is greater than 25%, the steeringstability improvement effect is reduced by the reduction in patternrigidity. Note that in order to obtain good steering stability whileensuring the water drainage properties, preferably, the groove arearatio GR is 10%≤GR≤22%, and more preferably 12%≤GR≤20%.

In the pneumatic tire as described above, the main grooves 11 may havethe same groove depth, or, they may have different groove depths fromeach other. In any case, when the maximum groove depth of the maingrooves 11 is GDmax (see FIG. 3), preferably 3.0 mm≤GDmax≤6.0 mm. Bysetting the maximum groove depth GDmax of the main grooves 11comparatively small in this way, the cornering force is increased, andthe steering stability can be improved. Here, if the maximum groovedepth GDmax of the main grooves 11 is smaller than 3.0 mm, the waterdrainage properties deteriorate, and conversely if greater than 6.0 mm,the steering stability improvement effect will be reduced.

In the pneumatic tire as described above, the fineness of the carcasscords of the carcass layer 4 can be selected from the range 900 dtex/2to 2000 dtex/2, for example, and the cord count per 50 mm unit width canbe selected from the range 30 to 70, for example. In particular, thefineness of the carcass cords of the carcass layer 4 may be 900 dtex/2to 1400 dtex/2, and the cord count per 50 mm unit width may be 45 to 70.In other words, adopting finer carcass cords contributes to making thecarcass layer 4 thinner and lighter, and on the other hand by increasingthe cord count of the carcass cords, the necessary pressure resistancecan be ensured. Here, if the fineness of the carcass cords is smallerthan 900 dtex/2 it is difficult to ensure the pressure resistance, andconversely if greater than 1400 dtex/2, the weight reduction effect isreduced.

In the pneumatic tire as described above, by providing at least oneinclined reinforcing layer 8 including a plurality of reinforcing cordsinclined with respect to the tire circumferential direction on the outercircumferential side of the carcass layer 4 in the tread portion 1, thecornering force is increased, and the steering stability can beimproved. In particular, if a plurality of inclined reinforcing layers 8laminated in the tire radial direction is provided on the outercircumferential side of the carcass layer 4 in the tread portion 1, thestiffness of the tread portion 1 in the tire width direction isincreased and the cornering force is effectively increased, and thesteering stability can be further improved. However, a structure inwhich the inclined reinforcing layer 8 is not provided in the treadportion 1 is also possible.

The inclination angle of the reinforcing cords of the inclinedreinforcing layer 8 with respect to the tire circumferential directionmay be from 15° to 60°. By appropriately adjusting the inclination angleof the reinforcing cords of the inclined reinforcing layer 8, sufficientcornering force can be ensured. Here, if the inclination angle of thereinforcing cords of the inclined reinforcing layer 8 with respect tothe tire circumferential direction is less than 15°, the amount ofincrease in the cornering power will be reduced. Conversely if theinclination angle is greater than 60°, the out-of-plane bending rigidityof the inclined reinforcing layer 8 will be reduced, so the rollingresistance will deteriorate due to the increase in ground contactlength. Note that if a plurality of inclined reinforcing layers 8 isprovided, preferably, the reinforcing cords of these inclinedreinforcing layers 8 are inclined with respect to the tirecircumferential direction between the layers in the opposite directionto each other. However, it is not necessary that the inclination anglewith respect to the tire circumferential direction be the same angle.Also, in order to achieve a good balance between cornering power androlling resistance, the inclination angle of the reinforcing cords ofthe inclined reinforcing layers 8 with respect to the tirecircumferential direction may be from 18° to 45°.

FIG. 5 and FIG. 6 are cross-sectional views illustrating modifiedexamples of the tread portion of the pneumatic tire of the presenttechnology. As illustrated in FIG. 5 and FIG. 6, at least onecircumferential reinforcing layer 9 extending along the tirecircumferential direction is arranged on the outer circumferential sideof the inclined reinforcing layer 8. In FIG. 5, a circumferentialreinforcing layer 9 includes a full cover that covers the whole area ofthe inclined reinforcing layer 8, and an edge cover that covers only theedge portions of the inclined reinforcing layer 8. In FIG. 6, thecircumferential reinforcing layer 9 includes only the edge cover thatcovers only the edge portions of the inclined reinforcing layer 8. Byproviding the circumferential reinforcing layer 9 on the outercircumferential side of the inclined reinforcing layer 8 in this way,the cornering power is increased, and the steering stability can beimproved.

Preferably, the circumferential reinforcing layer 9 is configured from acomposite material with reinforcing cords oriented in the tirecircumferential direction embedded in rubber. By increasing the in-planebending stiffness of the inclined reinforcing layer 8 by adding thecircumferential reinforcing layer 9 that includes reinforcing cordsoriented in the tire circumferential direction, the cornering power isincreased, and the steering stability can be improved. Note that a filmof thermoplastic resin or a thermoplastic elastomer composition that isa blend of a thermoplastic resin and an elastomer can be used as thecircumferential reinforcing layer 9.

The reinforcing cords of the circumferential reinforcing layer 9 may beorganic fiber cords. By using light organic fiber cords as thereinforcing cords of the circumferential reinforcing layer 9, the weightcan be reduced, and further the rolling resistance can be reduced.Ideally, organic fiber cords such as aramid, polyethylene terephthalate(PET), polyethylene naphthalate (PEN), nylon, and the like are used asthe reinforcing cords of the circumferential reinforcing layer 9, butmore preferably, high elasticity aramid fiber cords or composite cordswith high elasticity aramid fibers intertwined with low elasticity nylonfibers are used. Note that steel cords can also be used as thereinforcing cords of the circumferential reinforcing layer 9.

In the pneumatic tire as described above, the air penetration preventinglayer 7 is disposed in the tire interior and/or on the tire innersurface along the carcass layer 4, and the air penetration coefficientof the air penetration preventing layer 7 may be preferably 50×10⁻¹²cc·cm/cm²·sec·cmHg or less. In particular, the air penetrationpreventing layer 7 may be constituted from a thermoplastic resin or athermoplastic elastomer composition in which a thermoplastic resin andan elastomer are blended. By providing the air penetration preventinglayer 7 with a lower air penetration coefficient compared with aconventional air penetration preventing layer made from butyl rubber,the air penetration preventing layer 7 can be made thinner, so theweight can be further reduced. Here, if the air penetration coefficientof the air penetration preventing layer 7 is greater than 50×10⁻¹²cc·cm/cm²·sec·cmHg, it is difficult to further reduce the weight.

A thermoplastic resin or the thermoplastic resin composition in which athermoplastic resin and an elastomer are blended constituting the airpenetration preventing layer in the pneumatic tire of the presenttechnology will be described below.

Examples of the thermoplastic resin preferably used in the presenttechnology include: polyamide resins (for example, nylon 6 (N6), nylon66 (N66), nylon 46 (N46), nylon 11 (N11), nylon 12 (N12), nylon 610(N610), nylon 612 (N612), nylon 6/66 copolymers (N6/66), nylon 6/66/610copolymers (N6/66/610), nylon MXD6 (MXD6), nylon 6T, nylon 6/6Tcopolymers, nylon 66/PP copolymers, and nylon 66/PPS copolymers); theirN-alkoxyalkylated products (for example, methoxymethylated nylon 6,methoxymethylated nylon 6/610 copolymers, and methoxymethylated nylon612); polyester resins (for example, aromatic polyesters, such aspolybutylene terephthalate (PBT), polyethylene terephthalate (PET),polyethylene isophthalate (PEI), PET/PEI copolymers, polyarylate (PAR),polybutylene naphthalate (PBN), liquid crystal polyester, andpolyoxyalkylene diimide diacid/polybutylene terephthalate copolymers);polynitrile resins (for example, polyacrylonitrile (PAN),polymethacrylonitrile, acrylonitrile/styrene copolymers (AS),(meth)acrylonitrile/styrene copolymers, and(meth)acrylonitrile/styrene/butadiene copolymers); polymethacrylateresins (for example, polymethyl methacrylate (PMMA), polyethylmethacrylate); polyvinyl resins (for example, polyvinyl acetate,polyvinyl alcohol (PVA), vinyl alcohol/ethylene copolymers (EVOH),polyvinylidene chloride (PVDC), polyvinyl chloride (PVC), vinylchloride/vinylidene chloride copolymers, vinylidene chloride/methylacrylate copolymers, vinylidene chloride/acrylonitrile copolymers);cellulose resins (for example, cellulose acetate, and cellulose acetatebutyrate); fluororesins (for example, polyvinylidene fluoride (PVDF),polyvinyl fluoride (PVF), polychlorotrifluoroethylene (PCTFE), andethylene/tetrafluoroethylene (ETFE) copolymers); and imide resins (forexample, aromatic polyimide (PI)).

Examples of elastomers used in the present technology include dienerubbers and hydrogenated products thereof (for example, natural rubber(NR), isoprene rubber (IR), epoxidized natural rubber, styrene butadienerubber (SBR), butadiene rubber (BR, high-cis BR and low-cis BR), nitrilerubber (NBR), hydrogenated NBR, and hydrogenated SBR), olefin rubbers(for example, ethylene propylene rubber (EPDM, EPM), maleic acidmodified ethylene propylene rubber (M-EPM), butyl rubber (IIR),isobutylene and aromatic vinyl or diene monomer copolymer, acrylicrubber (ACM), and ionomer), halogen-containing rubbers (for example,Br-IIR, Cl-IIR, brominated copolymer of isobutylene/para-methyl styrene(Br-IPMS), chloroprene rubber (CR), chlorohydrin rubber (CHR),chlorosulfonated polyethylene rubber (CSM), chlorinated polyethylenerubber (CM), and maleic acid modified chlorinated polyethylene rubber(M-CM)), silicone rubbers (for example, methyl vinyl silicone rubber,di-methyl silicone rubber, and methyl phenyl vinyl silicone rubber),sulfur-containing rubbers (for example, polysulfide rubber),fluororubbers (for example, vinylidene fluoride rubbers,fluorine-containing vinyl ether rubbers, tetrafluoroethylene-propylenerubbers, fluorine-containing silicone rubbers, and fluorine-containingphosphazene rubbers), thermoplastic elastomers (for example, styreneelastomers, olefin elastomers, ester elastomers, urethane elastomers,polyamide elastomers), and the like.

If a particular thermoplastic resin among those described above isincompatible with such an elastomer, a compatibilizer may be used as athird component appropriately to make the two compatible with eachother. By mixing such a compatibilizer into the blend system, theinterfacial tension between the thermoplastic resin and the elastomer isreduced. As a result, the rubber particles constituting the dispersionphase is made finer, so that both components can exhibit theircharacteristics more effectively. In general, such a compatibilizer hasa copolymer structure of at least one of the thermoplastic resin and theelastomer, or a copolymer structure having an epoxy group, a carbonylgroup, a halogen group, an amino group, an oxazoline group, or ahydroxyl group, which is capable of reacting with the thermoplasticresin or the elastomer. The compatibilizer can be selected depending onthe type of the thermoplastic resin and the elastomer to be mixedtherewith. What is normally used is styrene/ethylene-butylene-styreneblock copolymers (SEBS) and their maleic acid-modified products, EPDM,EPM, EPDM/styrene or EPDM/acrylonitrile graft copolymers and theirmaleic acid-modified products, styrene/maleic acid copolymers, reactivephenoxine, and the like. The blending proportion of such acompatibilizer is not particularly limited, but may preferably be 0.5 to10 parts by weight relative to 100 parts by weight of the polymercomponents (the total amount of the thermoplastic resin and theelastomer).

In the thermoplastic elastomer composition, the component ratio of aparticular thermoplastic resin to a particular elastomer is notparticularly limited, and may be appropriately set so as to have astructure in which the elastomer is dispersed as a discontinuous phasein a matrix of the thermoplastic resin. However, the preferable range is90/10 to 15/85 in weight ratio.

In the present technology, the thermoplastic resin and the thermoplasticelastomer composition may be mixed with another polymer, for example,the above-described compatibilizer, in such an amount that the requiredcharacteristics as an air penetration preventing layer are not hindered.The purposes of mixing such a polymer are to improve the compatibilitybetween the thermoplastic resin and the elastomer, to improve themolding workability of the material, to improve the heat resistance, toreduce cost, and the like. Examples of the material used for the polymerinclude polyethylene (PE), polypropylene (PP), polystyrene (PS), ABS,SBS, and polycarbonate (PC). Furthermore, a reinforcing agent such as afiller (calcium carbonate, titanium oxide, alumina, and the like),carbon black, or white carbon, a softening agent, a plasticizer, aprocessing aid, a pigment, a dye, or an anti-aging agent that aregenerally compounded with polymer compounds may be optionally compoundedso long as the required characteristics as an air penetration preventinglayer are not hindered.

When mixed with the thermoplastic resin, the elastomer can bedynamically vulcanized. A vulcanizer, a vulcanization aid, vulcanizationconditions (temperature, time), and the like, during the dynamicvulcanization can be determined as appropriate in accordance with thecomposition of the elastomer to be added, and are not particularlylimited.

Generally available rubber vulcanizers (crosslinking agents) can be usedas the vulcanization agent. Specifically, as a sulfur-based vulcanizer,powdered sulfur, precipitated sulfur, highly dispersible sulfur, surfacetreated sulfur, insoluble sulfur, dimorpholine disulfide, alkylphenoldisulfide, and the like can be illustrated, and, for example,approximately 0.5 to 4 phr (in the present specification, “phr” refersto parts by weight per 100 parts per weight of an elastomer component;same below) can be used.

Moreover, examples of an organic peroxide-based vulcanizer includebenzoyl peroxide, t-butyl hydroperoxide, 2,4-dichlorobenzoyl peroxide,2,5-dimethyl-2,5-di(t-butyl peroxy)hexane,2,5-dimethylhexane-2,5-di(peroxyl benzoate), and the like. Such anorganic peroxide-based vulcanizer can be used in an amount of, forexample, around 1 to 20 phr.

Furthermore, examples of a phenol resin-based vulcanizer includebrominated alkylphenol resins and mixed crosslinking system containingan alkyl phenol resin with a halogen donor such as tin chloride andchloroprene. Such a phenol resin-based vulcanizer can be used in anamount of, for example, around 1 to 20 phr.

Examples of other vulcanizers include zinc oxide (approximately 5 phr),magnesium oxide (approximately 4 phr), litharge (approximately 10 to 20phr), p-quinone dioxime, p-dibenzoylquinone dioxime,tetrachloro-p-benzoquinone, poly-p-dinitrosobenzene (approximately 2 to10 phr), and methylenedianiline (approximately 0.2 to 10 phr).

As necessary, a vulcanization accelerator may be added. As thevulcanization accelerator, approximately 0.5 to 2 phr, for example, of agenerally available vulcanization accelerator of an aldehyde-ammoniabase, a guanidine base, a thiazole base, a sulfenamide base, a thiurambase, a dithio acid salt base, a thiourea base, or the like can be used.

Specific examples include an aldehyde ammonia vulcanization acceleratorsuch as hexamethylene tetramine and the like; a guanidine vulcanizationaccelerator such as diphenyl guanidine and the like; a thiazolevulcanization accelerator such as dibenzothiazyl disulfide (DM),2-mercaptobenzothiazole and its Zn salt, a cyclohexylamine salt, and thelike; a sulfenamide vulcanization accelerator such as cyclohexylbenzothiazyl sulfenamide (CBS), N-oxydiethylenebenzothiazyl-2-sulfenamide, N-t-butyl-2-benzothiazole sulfenamide,2-(thymol polynyl dithio)benzothiazole, and the like; a thiuramvulcanization accelerator such as tetramethylthiuram disulfide (TMTD),tetraethylthiuram disulfide, tetramethylthiuram monosulfide (TMTM),dipentamethylenethiuram tetrasulfide, and the like; a dithionatevulcanization accelerator such as Zn-dimethyl dithiocarbamate,Zn-diethyl dithiocarbamate, Zn-di-n-butyl dithiocarbamate,Zn-ethylphenyl dithiocarbamate, Te-diethyl dithiocarbamate, Cu-dimethyldithiocarbamate, Fe-dimethyl dithiocarbamate, pipecoline pipecolyldithiocarbamate, and the like; and a thiourea vulcanization acceleratorsuch as ethylene thiourea, diethyl thiourea, and the like.

Additionally, a vulcanization accelerator which is generally-used for arubber can be used. For example, zinc oxide (approximately 5 phr),stearic acid, oleic acid and their Zn salts (approximately 2 to 4 phr),or the like can be used.

The method for producing the thermoplastic elastomer composition is asfollows. The thermoplastic resin and the elastomer (unvulcanized one inthe case of rubber) are melt-kneaded in advance by a twin screw kneaderextruder or the like. The elastomer is dispersed as a dispersion phase(domain) in the thermoplastic resin forming a continuous phase (matrix).When the elastomer is vulcanized, the vulcanizer can be added during thekneading process to dynamically vulcanize the elastomer. Although thevarious compounding agents (except for the vulcanizer) may be added tothe thermoplastic resin or the elastomer during the kneading process, itis preferable to premix the compounding agents before the kneadingprocess. The kneader used for kneading the thermoplastic resin and theelastomer is not particularly limited. A screw extruder, kneader,Banbury Mixer, twin screw kneader extruder, or the like can be used asthe kneader. Among these, a twin screw kneader extruder is preferablyused for kneading the thermoplastic resin and the elastomer and fordynamically vulcanizing the elastomer. Furthermore, two or more types ofkneaders can be used to successively knead the thermoplastic resin andthe elastomer. As a condition for the melt-kneading, it is preferablethat a temperature be equal to or higher than a melting temperature ofthe thermoplastic resin. A shear rate when kneading is preferably from1000 to 7,500 sec−1. A total kneading time is from 30 seconds to 10minutes. Additionally, when a vulcanizing agent is added, avulcanization time after the addition is preferably from 15 seconds to 5minutes.

The polymer composition produced by the above method may be formed intoa desired shape by a generally-used method for forming a thermoplasticresin such as injection molding and extrusion molding.

The thermoplastic elastomer composition thus obtained has a structure inwhich the elastomer is dispersed as a discontinuous phase in the matrixof the thermoplastic resin. By having such a structure, it becomespossible to provide the inner liner layer with sufficient flexibilityand sufficient stiffness that is attributed to the effect of the resinlayer as a continuous phase. Furthermore, it becomes possible to obtain,during molding, a molding workability equivalent to that of thethermoplastic resin regardless of the amount of the elastomer.

The Young's modulus in a standard atmosphere of the thermoplastic resinand thermoplastic elastomer composition as set forth in JIS K7100 is notparticularly limited, but is preferably from 1 to 500 MPa, and morepreferably from 50 to 500 MPa.

The thermoplastic resin or the thermoplastic elastomer composition canbe formed into a sheet or film to be used as a single unit.Alternatively, an adhesive layer may be laminated thereon in order toimprove the adhesiveness to the adjacent rubber. Specific examples of anadhesive polymer that constitutes the adhesive layer include an ultrahigh molecular weight polyethylene (UHMWPE) having a molecular weight ofnot less than 1,000,000 and preferably not less than 3,000,000; acrylatecopolymers such as ethylene-ethylacrylate copolymers (EEA),ethylene-methylacrylate resins (EMA), and ethylene-acrylic acidcopolymers (EAA), and maleic anhydrate adducts thereof; polypropylene(PP) and maleic acid-modified products thereof; ethylene-propylenecopolymers and maleic acid-modified products thereof; polybutadieneresins and maleic anhydrate-modified products thereof,styrene-butadiene-styrene copolymers (SBS);styrene-ethylene-butadiene-styrene copolymers (SEBS); thermoplasticfluororesins; thermoplastic polyester resins; and the like. Thesepolymers can be formed into a sheet or film by being extruded with, forexample, a resin extruder in accordance with a generally-used method. Athickness of the adhesive layer is not particularly limited, but ispreferably small in order to reduce tire weight; and is preferably from5 μm to 150 μm.

EXAMPLES

Pneumatic tires were manufactured with a tire size of 195/65R15 or155/55R20, including a tread portion extending in the tirecircumferential direction in an annular shape; a pair of sidewallportions disposed on either side of the tread portion; a pair of beadportions disposed on the inside in the tire radial direction of thesidewall portions; one layer of a carcass layer disposed between thepair of bead portions; and an air penetration preventing layer on thetire inner surface. The ratio SW/OD of the tire total width SW and thetire outer diameter OD; the ratios SA/a, SB/b, and SC/c obtained fromthe cross-sectional areas SA, SB, SC (mm²) and the peripheral lengths a,b, and c (mm) of the first through third regions; the groove area ratioGR; the maximum groove depth GDmax of the main grooves; the inclinedreinforcing layer (number of layers, the inclination angle with respectto the tire circumferential direction of the reinforcing cords); thecircumferential reinforcing layer (presence or absence, form, material);and the air penetration preventing layer (material, air penetrationcoefficient) were varied as shown in Table 3 to Table 5 to produce aConventional Example, Comparative Examples 1 to 3, and Working Examples1 to 16.

The various test tires were evaluated for tire weight, fuel economyperformance, and steering stability, according to the followingevaluation methods; results are shown in Table 3 to Table 5.

Tire Weight

The weight of each test tire was measured. The evaluation results wereexpressed, using the inverse value of the measurement value, as an indexwith the Conventional Example being 100. Larger index values indicatelighter tire weight.

Fuel Economy Performance

Each test tire was assembled to a wheel of rim size 15×6J or 20×5J andfitted to a front wheel drive vehicle of engine displacement 1800 cc,inflated to an air pressure of 230 kPa, driven 50 laps of a test courseof total length 2 km at a speed of 100 km/h, and the fuel consumptionrate (km/L) was measured. The evaluation results were expressed as anindex with the Conventional Example being 100. Larger index valuesindicate superior fuel economy performance.

Steering Stability

Each test tire was assembled to a wheel of rim size 15×6J or 20×5J andfitted to a front wheel drive vehicle of engine displacement 1800 cc,inflated to an air pressure of 230 kPa, then three test drivers carriedout sensory evaluation for steering stability by driving three laps of atest course of total length 2 km while making lane changes, and theaverage value of the evaluation points from the test drivers wasobtained. The evaluation results were expressed as an index with theConventional Example being 100. Larger index values indicate superiorsteering stability.

TABLE 3 Comparative Comparative Conventional Example Example Example 1 2Tire size Nominal cross-sectional width 195 195 155 Nominal aspect ratio65 65 55 Nominal rim diameter 15 15 20 SW/OD 0.32 0.32 0.24 SA/a 13.49.4 14.5 SB/b 7.3 7.3 7.6 SC/c 10.0 10.0 10.0 Groove area ratio GR (%)30 30 30 Maximum depth of main grooves GDmax (mm) 6.8 6.8 6.8 InclinedNumber of layers 2 0 2 reinforcing Inclination angle (°) 20 — 20 layerCircumferential Presence or absence Present Absent Present reinforcingForm Cords — Cords layer Material Organic — Organic fibers fibersMaterial of air penetration preventing layer Rubber Rubber Rubber Airpenetration coefficient of air 100 100 100 penetration preventing layer(×10⁻¹² cc · cm/cm² · sec · cmHg) Tire weight (index) 100 110 105 FuelEconomy Performance (index) 100 101 100.5 Steering Stability (index) 10098 96 Comparative Working Working Working Example Example ExampleExample 3 1 2 3 Tire size Nominal cross-sectional width 155 155 155 155Nominal aspect ratio 55 55 55 55 Nominal rim diameter 20 20 20 20 SW/OD0.24 0.24 0.24 0.24 SA/a 9.7 9.7 9.7 9.7 SB/b 7.6 7.6 4.7 4.7 SC/c 10.010.0 10.0 6.0 Groove area ratio GR (%) 30 20 20 20 Maximum depth of maingrooves GDmax (mm) 6.8 6.8 6.8 6.8 Inclined Number of layers 0 0 0 0reinforcing Inclination angle (°) — — — — layer Circumferential Presenceor absence Absent Absent Absent Absent reinforcing Form — — — — layerMaterial — — — — Material of air penetration preventing layer RubberRubber Rubber Rubber Air penetration coefficient of air 100 100 100 100penetration preventing layer (×10⁻¹² cc · cm/cm² · sec · cmHg) Tireweight (index) 112 111 113 114 Fuel Economy Performance (index) 101.2101.1 101.3 101.4 Steering Stability (index) 94 103 101 100

TABLE 4 Working Working Working Working Example Example Example Example4 5 6 7 Tire size Nominal cross-sectional width 155 155 155 155 Nominalaspect ratio 55 55 55 55 Nominal rim diameter 20 20 20 20 SW/OD 0.240.24 0.24 0.24 SA/a 9.7 9.7 9.7 9.7 SB/b 4.7 4.7 4.7 4.7 SC/c 6.0 6.06.0 6.0 Groove area ratio GR (%) 20 20 20 20 Maximum depth of maingrooves GDmax (mm) 5.5 5.5 5.5 5.5 Inclined Number of layers 0 2 2 2reinforcing Inclination angle (°) — 20 20 20 layer CircumferentialPresence or absence Absent Absent Present Present reinforcing Form — —Film Cords layer Material — — Resin Steel Material of air penetrationpreventing layer Rubber Rubber Rubber Rubber Air penetration coefficientof air 100 100 100 100 penetration preventing layer (×10⁻¹² cc · cm/cm²· sec · cmHg) Tire weight (index) 114 113 113 112 Fuel EconomyPerformance (index) 101.4 101.3 101.3 101.2 Steering Stability (index)101 103 104 106 Working Working Working Example Example Example 8 9 10Tire size Nominal cross-sectional width 155 155 155 Nominal aspect ratio55 55 55 Nominal rim diameter 20 20 20 SW/OD 0.24 0.24 0.24 SA/a 9.7 9.79.7 SB/b 4.7 4.7 4.7 SC/c 6.0 6.0 6.0 Groove area ratio GR (%) 20 20 20Maximum depth of main grooves GDmax (mm) 5.5 5.5 5.5 Inclined Number oflayers 2 2 2 reinforcing Inclination angle (°) 20 20 20 layerCircumferential Presence or absence Present Present Present reinforcingForm Cords Cords Cords layer Material Organic Organic Organic fibersfibers fibers Material of air penetration preventing layer Rubber RubberResin Air penetration coefficient of air 100 50 10 penetrationpreventing layer (×10⁻¹² cc · cm/cm² · sec · cmHg) Tire weight (index)113 115 117 Fuel Economy Performance (index) 101.3 101.5 101.7 SteeringStability (index) 105 105 105

TABLE 5 Working Working Working Example Example Example 11 12 13 Tiresize Nominal cross-sectional width 155 155 155 Nominal aspect ratio 5555 55 Nominal rim diameter 20 20 20 SW/OD 0.24 0.24 0.24 SA/a 7.5 11.59.7 SB/b 2.0 6.0 4.7 SC/c 4.0 8.0 6.0 Groove area ratio GR (%) 20 20 10Maximum depth of main grooves GDmax (mm) 5.5 5.5 5.5 Inclined Number oflayers 2 2 2 reinforcing Inclination angle (°) 20 20 20 layerCircumferential Presence or absence Present Present Present reinforcingForm Cords Cords Cords layer Material Organic Organic Organic fibersfibers fibers Material of air penetration preventing layer Resin ResinResin Air penetration coefficient of air 10 10 10 penetration preventinglayer (×10−¹² cc · cm/cm² · sec · cmHg) Tire weight (index) 119 107 116Fuel Economy Performance (index) 101.9 100.7 101.6 Steering Stability(index) 102 107 106 Working Working Working Example Example Example 1415 16 Tire size Nominal cross-sectional width 155 155 155 Nominal aspectratio 55 55 55 Nominal rim diameter 20 20 20 SW/OD 0.24 0.24 0.24 SA/a9.7 9.7 9.7 SB/b 4.7 4.7 4.7 SC/c 6.0 6.0 6.0 Groove area ratio GR (%)25 20 20 Maximum depth of main grooves GDmax (mm) 5.5 3.0 6.0 InclinedNumber of layers 2 2 2 reinforcing Inclination angle (°) 20 20 20 layerCircumferential Presence or absence Present Present Present reinforcingForm Cords Cords Cords layer Material Organic Organic Organic fibersfibers fibers Material of air penetration preventing layer Resin ResinResin Air penetration coefficient of air 10 10 10 penetration preventinglayer (×10−¹² cc · cm/cm² · sec · cmHg) Tire weight (index) 117 116 117Fuel Economy Performance (index) 101.7 101.6 101.7 Steering Stability(index) 104 105 104

As can be seen from Table 3 to Table 5, compared with the ConventionalExample, it was possible to greatly reduce the tire weight of the tiresof Working Examples 1 to 16, and moreover it was possible to achieveboth steering stability and fuel economy performance.

On the other hand, with the tire of Comparative Example 1, in apneumatic tire with the ratio SW/OD of 0.32, the same as theConventional Example, the ratio SA/a was reduced, so although the tireweight reduction effect was obtained, the steering stability wasreduced. With the tire of Comparative Example 2, although the ratioSW/OD was reduced, the ratio SA/a was too large, so the fuel economyperformance improvement effect was insufficient. With the tire ofComparative Example 3, although the ratio SW/OD was small and the ratioSA/a was small, the groove area ratio GR was too large, so the steeringstability deteriorated greatly.

The invention claimed is:
 1. A pneumatic tire, comprising: a treadportion extending in a tire circumferential direction in an annularshape; a pair of sidewall portions disposed on either side of the treadportion; a pair of bead portions disposed on an inside in a tire radialdirection of the sidewall portions; and at least one layer of a carcasslayer disposed between the pair of bead portions; wherein a ratio SW/ODof a total tire width SW and a tire outer diameter OD satisfies arelationship SW/OD≤0.3, the total tire width SW being set in a rangefrom 125 mm to 185 mm and the tire outer diameter OD being set in arange from 650 mm to 850 mm; in a tire meridian cross-section, a contourof the tread portion that forms a tread profile includes side arcslocated on outermost sides in a tire width direction of the treadportion, and shoulder arcs located on an inner side in the tire widthdirection of the side arcs; a pair of first boundary lines is definedpassing through an intersection point of an extension line of the sidearcs and an extension line of the shoulder arcs and perpendicular to atire inner surface; each sidewall portion has a rim check line extendingin the tire circumferential direction; in a tire meridian cross-section,a pair of second boundary lines is defined passing through the rim checklines and perpendicular to the tire inner surface; a first region ispartitioned between the pair of first boundary lines; a second region ispartitioned between the first boundary line and a second boundary line;a third region is partitioned on a bead toe side of the second boundaryline; when cross-sectional areas (mm²) of the first region through thirdregion are SA, SB, and SC respectively, and peripheral lengths (mm) ofthe first region through third region along the tire inner surface area, b, and c respectively, a ratio SA/a satisfies a relationship7.5≤SA/a≤11.5; the tread portion has a tread pattern formed by grooves,and a groove area ratio GR in a ground contact region of the treadpattern is 25% or less; a linear density of carcass cords of the carcasslayer is from 1200 dtex/2 to 1300 dtex/2, and a cord count per 50 mmunit width of the carcass layer is from 55 to 70 cords; a bead filler isdisposed on an outer periphery side of each of the bead cores, a heightof the bead filler in the tire radial direction being less than a heightof the rim check line in the tire radial direction; and at least oneinclined reinforcing layer including a plurality of reinforcing cordsinclining with respect to the tire circumferential direction is disposedon an outer circumferential side of the carcass layer in the treadportion, an inclination angle of the reinforcing cords of the at leastone inclined reinforcing layer with respect to the tire circumferentialdirection being from 40° to 45°.
 2. The pneumatic tire according toclaim 1, wherein a ratio SB/b satisfies a relationship 2.0≤SB/b≤6.0. 3.The pneumatic tire according to claim 1, wherein a ratio SC/c satisfiesa relationship 4.0≤SC/c≤8.0.
 4. The pneumatic tire according to claim 1,wherein the grooves formed in the tread portion include at least onemain groove, and when a maximum groove depth of the main groove isGDmax, 3.0 mm≤GDmax≤6.0 mm.
 5. The pneumatic tire according to claim 1,wherein the groove area ratio GR in the ground contact region of thetread pattern satisfies 12%≤GR≤19%.
 6. The pneumatic tire according toclaim 1, further comprising an air penetration preventing layer with anair penetration coefficient of from 30×10⁻¹² cc·cm/cm²·sec·cmHg to50×10⁻¹² cc·cm/cm²·sec·cmHg in a tire interior and/or on the tire innersurface along the carcass layer.
 7. The pneumatic tire according toclaim 1, further comprising an air penetration preventing layer with anair penetration coefficient of 50×10⁻¹² cc·cm/cm²·sec·cmHg or less in atire interior and/or on the tire inner surface along the carcass layer.8. The pneumatic tire according to claim 7, wherein the air penetrationpreventing layer is formed from a thermoplastic resin or a thermoplasticelastomer composition in which a thermoplastic resin and an elastomerare blended.
 9. The pneumatic tire according to claim 1, wherein atleast one circumferential reinforcing layer extending along the tirecircumferential direction is arranged on the outer circumferential sideof the inclined reinforcing layer.
 10. The pneumatic tire according toclaim 9, wherein the circumferential reinforcing layer is configuredfrom a composite material with reinforcing cords oriented in the tirecircumferential direction embedded in rubber.
 11. The pneumatic tireaccording to claim 10, wherein the reinforcing cords of thecircumferential reinforcing layer are organic fiber cords.